Magnetic bubble memories are now well known in the art. Two well understood modes of operating bubble memories are the conductor-access mode and the field-access mode. A. H. Bobeck et al, U.S. Pat. No. 3,460,116 issued Aug. 5, 1969, describes a basic three-phase, conductor-access mode for operating bubble memories. A. H. Bobeck, U.S. Pat. No. 3,534,347, issued Oct. 13, 1970, described the basic field-access mode for operating bubble memories. We are concerned primarily with the conductor-access mode herein.
The basic conductor-access mode bubble memory is hard to realize commercially. The primary reason for this is that three-phase operation is required to impart unidirectional movement to bubbles and thus a three-phase propagation drive arrangement is required. The basic conductor-access arrangement thus comprised three separate levels of metallization each having a serpentine electrical conductor pattern. The patterns are offset from one another and pulsed in a three-phase manner to realize proper bubble movement. Such circuits are characterized by the following problems: Firstly, since current is impressed from the end of a long conductor; high power consumption is the rule. Further, although the conductor levels are separated by insulating layers, the resulting circuits are subject to short circuits which are fatal. Moreover, nonuniformities in the conductor pattern result in current nonuniformities which cause a loss in operating margins. Also, multilevel conductor patterns are costly to realize.
R. F. Fischer, U.S. Pat. No. 3,564,518, issued Feb. 16, 1971, discloses the use of structured-in rest positions operative as a phase of a multiphase conductor-access arrangement. The rest positions are defined by patterns of permalloy dots a pair of which is offset from each conductor of a two-phase conductor pattern. Specifically, each of two conductors of the pattern is pulsed in sequence to move a bubble, in each instance, to a position offset from a rest position. At the termination of a current pulse, the bubble offsets to the associated pair of permalloy dots thus ensuring unidirectional movement of the bubble. The use of the permalloy pattern reduces the requisite number of metallization levels to two and reduces power requirements but adds a permalloy level.
U.S. Pat. Nos. 3,693,177 and 3,678,479 issued Sept. 19, 1972 and July 18, 1972, respectively, disclose magnetic bubble memories which even further reduce the number of metallization levels to one. Here again the use of offset rest positions for bubbles is provided for to reduce the complexity of the conductor implementation. In the memories of these patents, two propagation phases are realized with a single metallization by applying bipolar pulses to the metallization. The underlying bubble layer itself is formed into undulating strips to define the rest positions and to provide interpath decoupling.
The problem with the conductor-access arrangements with single-metallization-level implementations is that they are still high power devices, intolerant of faults, and demanding on photolithographic processing.